Fluid swivels are commonly used in offshore installations to transfer gas and oil between a fixed underwater pipeline and a tanker that may drift around the installation. A typical fluid swivel includes ring-shaped inner and outer walls, or structures, forming an annular chamber between them, and forming a pair of gap passages extending from opposite sides of the chamber to the environment. A pressure seal is placed along each gap passage. A common type of pressure seal is a radial seal with opposite sides that press against radially spaced surfaces as one structure (e.g. the outer structure) slowly rotates about the other one. Although face seals are sometimes used which seal against axially-spaced surfaces, radial seals are often preferred because they commonly result in fewer separate parts that are easier to machine, and because fluid swivels with radial seals are generally easier to design.
The radial pressure seal has an upstream side exposed to the same pressure as that which exists in the annular chamber, and has a downstream side which is generally at ambient pressure (one atmosphere). Where fluid in the annular chamber is at moderate to high pressure, there is a tendency for the pressure seal to extrude into the portion of the gap passage lying immediately downstream of the pressure seal, which can be referred to as the "extrusion gap". To avoid extrusion, the extrusion gap is made as narrow as possible.
When high pressure fluid lies in the annular chamber formed between the inner and outer structures, the outer structure tends to slightly increase in diameter, while the inner structure tends to slightly decrease in diameter. Thus, the high pressure fluid tends to separate the inner and outer structures, which increases the thickness of the gap passage, including the extrusion gap. Any increase in thickness of the extrusion gap can result in a significant decrease in the life of the pressure seal. It may be noted that it is possible to orient the extrusion gap so it extends radially, but this can complicate construction of the fluid swivel. A fluid swivel that used radial seals and which was designed to hold high pressure fluid (at at least 50 atmospheres), which minimized changes in the width of the extrusion gap which lies immediately downstream of a pressure seal, would be of considerable value.